virpci: Resolve leak in virPCIVirtualFunctionList cleanup

Repeatedly querying an SR-IOV PCI device's capabilities exposes a
memory leak caused by a failure to free the virPCIVirtualFunction
array within the parent struct's g_autoptr cleanup.

Valgrind output after getting a single interface's XML description
1000 times:

==325982== 256,000 bytes in 1,000 blocks are definitely lost in loss record 2,634 of 2,635
==325982==    at 0x4C3C096: realloc (vg_replace_malloc.c:1437)
==325982==    by 0x59D952D: g_realloc (in /usr/lib64/libglib-2.0.so.0.5600.4)
==325982==    by 0x4EE1F52: virReallocN (viralloc.c:52)
==325982==    by 0x4EE1FB7: virExpandN (viralloc.c:78)
==325982==    by 0x4EE219A: virInsertElementInternal (viralloc.c:183)
==325982==    by 0x4EE23B2: virAppendElement (viralloc.c:288)
==325982==    by 0x4F65D85: virPCIGetVirtualFunctionsFull (virpci.c:2389)
==325982==    by 0x4F65753: virPCIGetVirtualFunctions (virpci.c:2256)
==325982==    by 0x505CB75: virNodeDeviceGetPCISRIOVCaps (node_device_conf.c:2969)
==325982==    by 0x505D181: virNodeDeviceGetPCIDynamicCaps (node_device_conf.c:3099)
==325982==    by 0x505BC4E: virNodeDeviceUpdateCaps (node_device_conf.c:2677)
==325982==    by 0x260FCBB2: nodeDeviceGetXMLDesc (node_device_driver.c:355)

Signed-off-by: Tim Shearer <tshearer@adva.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>